Motion-picture projectors use effectively exclusively xenon high-pressure discharge lamps having power ratings of between about 0.5 to 10 kW. In micro lithography, mercury high-pressure discharge lamps are used with power ratings of between about 0.2 kW and 2 kW. Lamps for both uses are direct current operated, and the light is generated by a d-c arc of high stability extending between a cathode and an anode. The thermal loading on the anode is extremely high.
It is believed that the thermal loading on the anode is caused primarily by entry of electrons during the discharge into the facing end surface of the anode. Usually, the anode has a blunt end surface, and typically a somewhat spherical or frusto-conical end region. Upon entry of electrons, anode heat is generated and has to be dissipated. This heat is caused, in part, by the entrance work or entrance or insertion energy (about 4.4 eV for tungsten), the anode drop (about 1 eV) and the average plasma electron energy (about 1 eV). This heat increases in proportion to the lamp current.
Anodes for such high-pressure discharge lamps usually are made of tungsten, the metal of all metals which has the highest melting point of about 3680 K., and the lowest vapor pressure. The anodes are as large as possible, while still assuring a stable arc. The size should be so selected that it provides the largest possible heat radiating surface in order to obtain lowering of the anode operating temperature. Of course, due to costs and design considerations, as well as size of the overall lamp, the size of the anode cannot be increased beyond relatively narrow limits.
To obtain a low anode end surface temperature which is as low as possible, it is desirable to also provide for effective conduction of anode heat away from the anode front end surface. This requires a high heat conductivity of the tungsten material in the region of the front end surface.
The tungsten material used in making the anode must have high resistance with respect to material creep due to heat. This is particularly important due to the high anode front end surface temperatures. High mechanical strength at high temperature over a long time is expected so that the anode surface does not deform in the region facing the arc. Such deformation may lead to premature failure of the lamp.
Anodes which have insufficient resistance to high temperature creep may generate craters or the like in the region of the front end surface. Additionally, heating and cooling processes occur when the lamp, when cold, is energized, and then, after deenergization, cools again. The resulting thermal stresses may cause fissures at which sooner or later local hot spots will occur during lamp operation which may reach temperatures above the melting point of the tungsten. This leads to increased vaporization of tungsten and thus premature blackening of the discharge vessel, again reducing the effective lifetime of the lamp.
Usually, the tungsten for use in anodes utilizes tungsten rod or stock material which is made in a powder metallurgy process. Pure or doped tungsten powder is compressed into rod shape, and sintered; the thus premanufactured rods are circumferentially swaged, in which the sintered tungsten rod material is reformed and compacted.
The two important parameters for long life of anodes are the heat conduction characteristics and the high temperature creep resistance characteristics. Both of these parameters increase with compaction of the tungsten material. It is believed that the high temperature creep resistance increases not only due to the higher density of the material, but also due to the finer grains of the material which arise upon compaction. The larger numbers of grain boundaries within a unit volume then counteract thermally caused material deformations or shifts and provide greater resistance against material shifting under heat loading.
The deformation and compaction process acting on rod material are hammer forces which act essentially radially on the rod material. Deformation and compaction is done from the lateral surface. As a result, both the high temperature creep resistance and the heat conductivity decrease from the outer or sleeve surface of the rod material towards the center of the rod material.
When a lamp is energized, the arc attaches at about the center region of the anode face surface. That is the region where the highest temperature results. That is also the region where the compaction in accordance with the prior art processes has a minimum effect. Consequently, that central region of the anode has the lowest density, hence the lowest heat conductivity, and the lowest resistance towards creep under high temperature loading.